1. Field of Invention
The invention relates to an optical recording medium and a method for making the same. More particularly, the invention relates to a digital versatile disc-recordable (DVD-R) and a method for making the same.
2. Description of Related Art
As the digital era progresses, more and more multimedia and network products are becoming known, as the consumer's request to store computer data increases. Considering storage capacity, safety and convenience, the digital versatile disc-recordable (DVD-R) is the most useful tool for storing a large amount of data such as video data. The development of DVD-R is highly appreciated, since its manufacturing process is similar to that of the compact disc and lower cost.
The manufacturing process of DVD-R is based on the technique of the compact disc. A molded plastic substrate with a thickness of 0.6 mm having grooves thereon is first provided. On the basis of low cost, low moisture absorption, high transparency and high throughput, the PC (polycarbonate) substrate is preferred. Thereafter, a dye layer serving as a recording layer is coated on the substrate by spin-coating. The dye layer has a maximum absorption at the wavelength range from 500 nm to 650 nm. A metal layer serving as a reflection layer is formed on the dye layer. Another PC protective layer having a thickness of 0.6 mm is bounded to the reflection layer to form a disc having a thickness of 1.2 mm. However, in the process of manufacturing the CD-R, after a reflection layer is formed on a substrate with a thickness of 1.2 mm by sputtering, an ultraviolet setting resin is spin-coated on the reflection layer. The difference between CD-R and DVD-R is that DVD-R is formed by two PC plates, each having a thickness of 0.6 mm; while CD-R is formed by only one PC plate having a thickness of 1.2 mm.
In the spin-coating process of CD-R or DVD-R, dye is uniformly coated on the substrate. The amount of the dye in the grooves and on the lands between grooves is adjusted by the parameter of the spin-coating process so that the light path difference (Ld) between the grooves and the lands can be controlled. The light path difference between the groove and the land affects the tracking signal and compatibility and then the recording quality of the disk is affected.
In theory, the depth and the width of the grooves can be designed and the solvent or the dye can be selected to control the light path difference. However, this still has limitations. First, the depth of the grooves cannot be increased or decreased arbitrarily. If the depth of the grooves increases, the aspect ratio increases so that the size of the grooves cannot be precisely controlled during the molding process of the substrate. If the depth of the grooves decreases, recorded signal modulation of the disk products are too low, so as to decrease the recording quality. On the other hand, the substrate is usually a PC plate, but the solvent resistance of PC material is poor. Therefore, the dyes usually dissolve in alcohol such as tetrafluoropropanol. However, the solubility of azo metal chelate compounds in alcohol is low. Specifically, when the solubility is below 2%, the gap filling property of the dyes in the grooves is poor, so that a tracking signal such as the push-pull tracking signal amplitude before cannot conform to the standard and so the compatibility of the discs is affected. Therefore, if the solubility of the dyes is too low, it cannot be used in the industry.
Japanese Unexamined Patent Publications No. 112790/1983, No. 114989/1983, No. 85791/1984 and No. 83236/1985 provide an optical recording medium using cyanine type dyes as the dye layer. The cyanine type dyes have high solubility, so that alcohol can be used as a solvent to dissolve it and the cyanine type dyes have a good ability to fill the grooves.
However, the cyanine type dye has poor light resistance. When the cyanine type dye is applied to the optical recording medium, quencher is required to be added for providing a sufficient durability against the optical degradation, for example, U.S. Pat. No. 5,328,741, U.S. Pat. No. 5,328,802 and U.S. Pat. No. 5,336,584. The quencher can improve the light resistance, but reducing solubility during the process for fabricating the disc is a drawback of the quencher. If the quencher is not appropriately used, crystal is precipitated so as to damage the recording function. On the other hand, the cost of the disc increases since the quencher is more expensive than the cyanine type dye.